If I had to pick a standout improvement in EE2 it would be, without question, the improved 3D models within the game and the excellent texturing done to them. EECH was always a nice looking sim in my opinion, but the higher resolution textures and rounding off of the angular corners on many of the vehicles is a very worthwhile improvement. Here is a short gallery of some comparison shots between EECH and EE2 models. Not all of them are better (UH-60 and AH-64D 3D models for example), but compare for yourself.
EECH images are on the left, EE2 images on the right. Click a thumbnail to see the enlarged image.